Scanning for emails containing credit card numbers can bounce back with a custom PCI DSS compliance notification. Here is an example email that I got from someone.

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Due to PCI DSS Compliance (Payment Card Industry Data Security Standard), we are unable to accept emails that contain credit card information. Under requirement #3 we need to Protect Cardholder Data; email is not a secure transfer of data.
Please contact your account executive via phone to provide your credit card information. If you have previously used a credit card for your purchases and that card number is on file with us, simply send your account executive an email containing just the last 4 digits of that credit card.

For more information on PCI DSS Compliance, please visit the following website:
https://www.pcisecuritystandards.org/security_standards/index.php
If you feel you have received this message in error, please contact your account executive.
Credit Card Security Department
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